1. Field of the Invention
The present invention relates to an organic electroluminescence element used for a light emission device in various apparatuses and an exposure unit as well as an image-forming apparatus using the element.
2. Description of the Related Art
An electroluminescence element is a light emission device using electric field-induced light emission of a solid luminescent material. Now, inorganic electroluminescence elements that use inorganic materials as the light emitter are in practical use, and expansion of their applications to the backlight for liquid crystal displays or flat panel displays are intended in some segments. However, the voltage required for the light emission of inorganic electroluminescence elements is rather high, i.e., 100 v or higher. In addition, due to the difficulty in blue light emission, it is difficult to achieve full color emission based on the three primary colors of R, G and B. Moreover, since the refractive index of the material used as the light emitter of an inorganic electroluminescence element is very high, the emission light is strongly affected by the effect of the total reflection at boundaries. Accordingly, the efficiency of taking out the actually emitted light into the air is as low as roughly 10 to 20%, which value is difficult to improve.
On the other hand, studies on electroluminescence elements using organic materials have called attention for a long time. Though various investigations have been made, they never evolved to a full-scale study for practical use because the emission efficiency was extremely low.
But, in 1987, C. W. Tang et al of Kodak Co. proposed an organic electroluminescence element having a function-separated, stacked structure in which the organic material is divided into two layers, i.e., a hole transport layer and a light emission layer. And it has become evident that, in spite of a low voltage of 10 V or lower, an emission luminance as high as 1000 cd/m2 or more is attained (Refer to C. W. Tang and S. A. Vanslyke; Applied Physics Letter (Appl. Phys. Lett.) (USA), Vol. 51, 1987, p. 913.). Since then, organic electroluminescence elements have attracted attention on a sudden. Still now, function separation type organic electroluminescence elements having a similar stacked structure are being actively studied. In particular, efficiency enhancement and life expansion, which are indispensable for the product development of organic electroluminescence elements, are also being thoroughly investigated, resulting in the recent development of displays using organic electroluminescence elements.
Now, the structure of a conventional, common organic electroluminescence element will be explained with reference to FIG. 21. FIG. 21 is a cross-sectional view showing the essential part of a conventional organic electroluminescence element.
As is shown in FIG. 21, the organic electroluminescence element includes an anode 52 comprising a transparent electro-conductive film such as ITO formed by sputtering or resistive heating vapor deposition on a substrate 51 made of, for example glass, a hole transport layer 53 made of, for example, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (which will be abbreviated as TPD hereinafter) similarly formed by resistive heating vapor deposition on the anode 52, a light emission layer 54 made of aluminum 8-hydroxyquinoline (which will be abbreviated as Alq3 hereinafter) prepared by resistive heating vapor deposition on the hole transport layer 53, and a cathode 55 made of a metallic film with a thickness of 100 to 300 nm formed by resistive heating vapor deposition on the light emission layer 54.
When a dc voltage or dc current is applied to the organic electroluminescence element having such structure by making the anode 52 a positive electrode and the cathode 55 a negative electrode, holes are injected into the light emission layer 54 from the anode 52 via the hole transport layer 53, and electrons are injected into the light emission layer 54 from the cathode 55. In the light emission layer 54, recombination of the hole and electron takes place; and when an exciton generated by such recombination shifts from the excited state to the ground state, the phenomenon of light emission takes place.
Generally speaking, in such organic electroluminescence element, the light emitted from the luminescent-material in the light emission layer 54 radiates omni-directionally from the luminescent material as the center, and emerges into the air through the hole transport layer 53, anode 52 and substrate 51. Alternatively, the light once proceeds in the direction opposite to the light emerging direction (the direction toward the substrate 51), is reflected at the cathode 55, and emerges into the air through the light emission layer 54, hole transport layer 53, anode 52 and substrate 51.
Regarding the device structure of organic electroluminescence elements, there are some descriptions set forth in U.S. Pat. No. 5,917,280 and U.S. Pat. No. 5,932,895.
In an image-forming apparatus based on electrophotographic technology, an exposure unit is provided which irradiates exposure light corresponding to image data onto a photoreceptor having been charged to a pre-determined uniform potential and records an electrostatic latent image on the photoreceptor. As the conventional exposure method for such exposure unit, those based on laser beams or LED arrays are dominant.
In the case of laser beam exposure, downsizing of the unit is quite difficult since optical parts such as a polygon mirror or lenses occupy large spaces. In the case of LED array exposure, cost reduction of the unit is difficult because the circuit board is expensive.
Now, with use of the above-described organic electroluminescence element as the light source, these problems can be solved.
However, since the light emitted from the organic electroluminescence element is diffusive, it has been impossible to achieve a sufficient level of light quantity required to form an image on a photoreceptor with the diffusive light from the conventional element.